Direct Experimental Probe of the Ni(II)/Ni(III)/Ni(IV) Redox Evolution in LiNi0.5Mn1.5O4 Electrodes

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• 作者：Ruimin Qiao ; L. Andrew Wray ; Jung-Hyun Kim ; Nicholas P. W. Pieczonka ; Stephen J. Harris ; Wanli Yang
• 刊名：Journal of Physical Chemistry C
• 出版年：2015
• 出版时间：December 10, 2015
• 年：2015
• 卷：119
• 期：49
• 页码：27228-27233
• 全文大小：378K
• ISSN：1932-7455

文摘

The LiNi_0.5Mn_1.5O₄ spinel is an appealing cathode material for next generation rechargeable Li-ion batteries due to its high operating voltage of 鈭?.7 V (vs Li/Li⁺). Although it is widely believed that the full range of electrochemical cycling involves the redox of Ni(II)/(IV), it has not been experimentally clarified whether Ni(III) exists as the intermediate state or a double-electron transfer takes place. Here, combined with theoretical calculations, we show unambiguous spectroscopic evidence of the Ni(III) state when the LiNi_0.5Mn_1.5O₄ electrode is half charged. This provides a direct verification of single-electron-transfer reactions in LiNi_0.5Mn_1.5O₄ upon cycling, namely, from Ni(II) to Ni(III), then to Ni(IV). Additionally, by virtue of its surface sensitivity, soft X-ray absorption spectroscopy also reveals the electrochemically inactive Ni²⁺ and Mn²⁺ phases on the electrode surface. Our work provides the long-awaited clarification of the single-electron transfer mechanism in LiNi_0.5Mn_1.5O₄ electrodes. Furthermore, the experimental results serve as a benchmark for further spectroscopic characterizations of Ni-based battery electrodes.